1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling Semi-Persistent Scheduling (SPS) transmission in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as to user equipments (UEs).
The LTE system has two scheduling methods: dynamic scheduling (DS) and semi-persistent scheduling (SPS). For DS, the network dynamically allocates resources to UEs for data reception or transmission depending on traffic volume, quality of service (QoS) requirements of each UE. And for SPS, the network periodically allocates an SPS resource to UEs, in order to serve upper layer applications which generate semi-static size data periodically, e.g. Voice over Internet Protocol (VoIP) services, for reducing control information sent on a physical downlink control channel (PDCCH) and enhancing system scheduling performance. In other words, SPS provides semi-persistent transmission resources, i.e. configured UL grant, to the UE, such that the UE can perform periodic data transmission without receiving PDCCH.
A long term evolution-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at a cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (CoMP), UL multiple-input multiple-output (MIMO), etc.
For bandwidth extension, carrier aggregation is introduced to the LTE-A system by which two or more component carriers are aggregated to achieve a wider-band transmission. Accordingly, the LTE-A system can support a wider bandwidth up to 100 MHz by aggregating a maximum number of 5 component carriers, where bandwidth of each component carrier is 20 MHz and is backward compatible with 3GPP Rel-8. An LTE-A specification supports carrier aggregation for both continuous and non-continuous component carriers with each component carrier limited to a maximum of 110 resource blocks. The carrier aggregation increases a bandwidth flexibility by aggregating the non-continuous component carriers. A component carrier is used as an UL component carrier or a downlink (DL) component carrier. Further, there is a one-to-one correspondence between the UL component carrier and the DL component carrier, i.e., each UL component carrier is paired with a corresponding DL component carrier. In an LTE-A time-division duplex (TDD) system, the UL component carrier and DL component carrier are the same component carrier.
When the UE is configured with the carrier aggregation (CA), the UE is allowed to receive and transmit data on one or multiple component carriers to increase the data rate. In the LTE-A system, it is possible for the eNB to configure the UE different numbers of UL and DL component carriers which depend on UL and DL aggregation capabilities, respectively. Moreover, the component carriers configured to the UE necessarily consists of one DL primary component carrier (PCC) and one UL primary component carrier. Component carriers other than the primary component carriers are named UL or DL secondary component carriers (SCCs). The numbers of UL and DL secondary component carriers are arbitrary, and are related to the UE capability and available radio resource. The PCell can not be de-activated, but can be changed by a handover procedure with the RACH procedure. In carrier aggregation, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the NAS mobility information, and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as the Primary Cell (PCell). In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC).
According to the specification released by 3rd Generation Partnership Project (3GPP), the UE shall clear the configured uplink grant immediately after implicitReleaseAfter [8] number of consecutive new medium access control (MAC) Protocol Data Units (PDUs) each containing zero MAC service data units (SDUs) have been provided by the Multiplexing and Assembly entity, on the Semi-Persistent Scheduling resource. As a result, the Semi-Persistent Scheduling resource can be released when the UE does not have data for transmission.
However, assume that the UE is configured with a primary component carrier, at least one secondary component carrier (i.e. corresponding to a PCell and at least a SCell, respectively), a Semi-Persistent Scheduling (SPS) resource on the primary component carrier and an implicitReleaseAfter value set to 2. In a first subframe (i.e. a transmission time interval (TTI)), the UE has a first dynamic grant and a first configured grant for transmission. The UE does not have enough data for uplink transmission using both the first dynamic grant and the first configured grant, so the UE transmits a first MAC PDU containing data using the first dynamic grant and a second MAC PDU containing no data (i.e. zero MAC SDUs) using the first configured grant. In a second subframe, the UE has a second dynamic grant and a second configured grant. The UE does not have enough data for uplink transmission using both the second dynamic grant and the second configured grant, so the UE transmits a third MAC PDU containing data using the second dynamic grant and a forth MAC PDU containing no data (i.e. zero MAC SDUs) using the second configured grant.
Under such a situation, since the UE transmits two consecutive MAC PDUs each containing zero MAC SDUs, the UE clears the Semi-Persistent Scheduling resource while the UE actually has data for transmission. Therefore, in carrier aggregation, since the UE may transmit data using dynamic grants rather than configured grants, it is more likely for the UE to transmit consecutive MAC PDUs each containing zero MAC SDUs using the configured grants and thus clear the Semi-Persistent Scheduling resource, which wastes PDCCH resources for frequent SPS activation due to frequent SPS implicitly deactivation.